For example, characteristics of secondary batteries such as lead-acid batteries vary with the temperature of the electrolytic solution, the electrode plates, and the like, that is, the internal temperature. As such, there is a need to accurately find the internal temperature in order to accurately detect a state of the secondary battery. Particularly, for a secondary battery installed in a vehicle, the internal temperature changes significantly due to the heat generated by the engine and the like. Therefore, detection of the internal temperature is essential to accurately detect the state of the secondary battery.
Conventionally, techniques such as those described, for example, in Patent Document 1 and Patent Document 2 have been used as techniques for finding the internal temperature of a secondary battery.